Short answer: it's closely related to the Content-Security-Policy: upgrade-insecure-requests response header, indicating that the browser supports it (and in fact prefers it).
It took me 30mins of Googling, but I finally found it buried in the W3 spec.
The confusion comes because the header in the spec was HTTPS: 1, and this is how Chromium implemented it, but after this broke lots of websites that were poorly coded (particularly WordPress and WooCommerce) the Chromium team apologized:
"I apologize for the breakage; I apparently underestimated the impact based on the feedback during dev and beta."
— Mike West, in Chrome Issue 501842
Their fix was to rename it to Upgrade-Insecure-Requests: 1, and the spec has since been updated to match.
Anyway, here is the explanation from the W3 spec (as it appeared at the time)...
The
HTTPSHTTP request header field sends a signal to the server expressing the client’s preference for an encrypted and authenticated response, and that it can successfully handle the upgrade-insecure-requests directive in order to make that preference as seamless as possible to provide....
When a server encounters this preference in an HTTP request’s headers, it SHOULD redirect the user to a potentially secure representation of the resource being requested.
When a server encounters this preference in an HTTPS request’s headers, it SHOULD include a
Strict-Transport-Securityheader in the response if the request’s host is HSTS-safe or conditionally HSTS-safe [RFC6797].